1. Field of the Invention
The present invention relates to communications systems, and, in particular, to discrete multi-tone (DMT) telecommunications systems.
2. Description of the Related Art
DMT systems transmit information encoded in multiple tones, where each tone corresponds to a modulated carrier signal having a different frequency. DMT systems conforming to the ITU G.992.1 (also known as G.dmt) standard use 255 different tones corresponding to a bandwidth of 1.104 MHz. Such DMT systems are referred to herein as full-rate DMT systems. DMT systems conforming to the ITU G.992.2 (also known as G.lite) standard use 127 different tones corresponding to a bandwidth of 0.552 MHz. Such DMT systems are referred to herein as half-rate DMT systems. The first 127 tones of the 255-tone G.dmt standard (i.e., tones #1-127) have the same carrier frequencies as the 127 tones of the G.lite standard. The tones (i.e., sub-carriers) are at i*F KHz, where i=1, . . . , 255 and F=4.3125 KHz is the frequency spacing between tones. Note that the set of G.lite tones is different from the set of G.dmt tones even though they have tones in common (i.e., tones #1-127), because at least one set (i.e., the set of G.dmt tones) has one or more tones (i.e., tones #128-255) that are not in the other set (i.e., the set of G.lite tones).
In typical distributed DMT telecommunications systems based on either the G.dmt or G.lite standard, tones #1-31 are reserved for upstream transmissions from the client (e.g., consumer personal equipment (CPE)) to the centralized facility (e.g., central office (CO)), while the remaining tones (i.e., tones #32-255 for full-rate DMT systems and tones #32-127 for half-rate DMT systems) are reserved for downstream transmissions from the CO to the client. If echo cancellation is supported at both the CO and the client, then the DMT system can operate in an overlapped spectrum mode, where tones #1-31 are used for both upstream and downstream transmissions. General conventional DMT transceivers are described in further detail in John Cioffi, The communications handbook, Chapter 34, xe2x80x9cAsymmetric Digital Subscriber Lines,xe2x80x9d IEEE Press 1997, the teachings of which are incorporated herein by reference.
At the beginning of each connection, certain initialization operations (e.g., transceiver training, channel analysis, message exchange, and bit loading) are performed so that each node (i.e., the client and the CO) can select which of the available tones it wishes the transmitter of the other node to use during datamode (i.e., xe2x80x9cnon-initializationxe2x80x9d) transmissions. As part of these initialization operations, the CO transmits initialization signals, such as reverb and medley, containing all of the downstream tones to the client, and the client selects the one or more particular tones that the client wishes the CO""s transmitter to use for datamode downstream transmissions. Thus, during initialization operations in a full-rate DMT system, the CO transmits up to 255 tones to the client and the client responds by selecting a subset of those 255 tones. Similarly, in half-rate DMT systems, the CO transmits up to 127 tones to the client and the client responds by selecting a subset of those 127 tones. After initialization operations are complete, the CO will use the tones selected by the client for datamode downstream transmissions (and the client will use tones selected by the CO for datamode upstream transmissions).
Because the bandwidth of full-rate DMT systems is twice that of half-rate DMT systems (i.e., 1.104 MHz vs. 0.552 MHz), the circuitry used to generate and/or process signals in full-rate DMT systems is typically more complex than that used for half-rate DMT systems. In particular, the minimum sampling rate by a receiver in a full-rate DMT system is twice the minimum sampling rate by a receiver in a half-rate DMT system (e.g., 2.208 MHz vs. 1.104 MHz). The higher sampling rate results in greater circuit complexity in various other components of a full-rate DMT receiver, such as the time-domain equalizer (TDQ), the echo canceller, and the fast Fourier transform (FFT) processor. The TDQ is a digital filter that is trained to shorten the impulse response of the channel to a length less than the length of the cyclic prefix. The echo canceller removes interference from the transmit signal to the receive band. It can be used advantageously in the non-overlapped spectrum case to provide substantial performance improvement. The FFT converts the time-domain signal to a frequency-domain signal. Due to the higher sampling rate of the full-rate DMT system as compared to the half-rate DMT system, an FFT in a full-rate DMT receiver is typically at least twice as complex as an FFT in a half-rate DMT receiver (e.g., a 512-point FFT vs. a 256-point FFT).
The present invention is directed to a technique for implementing low-complexityxe2x80x94and therefore low-costxe2x80x94DMT transceivers in full-rate DMT communications systems. According to embodiments of the present invention, a slightly modified, half-rate DMT transceiver conforming to the ITU G.992.2 (G.lite) standard can be connected to a full-rate DMT system conforming to the ITU G.992.1 (G.dmt) standard. The prior art does not support such interconnection of components conforming to different DMT standards. As such, the present invention enables the use of low-cost DMT equipment in already deployed full-rate DMT systems.
In one embodiment, the present invention is a method for processing a received discrete multi-tone (DMT) signal conforming to a first DMT standard based on a first set of tones, comprising the steps of (a) sampling and filtering the received DMT signal to generate a filtered, sampled DMT signal; and (b) further processing the filtered, sampled DMT signal based upon a second DMT standard based on a second set of tones different from the first set of tones.
In another embodiment, the present invention is an apparatus for processing a received DMT signal conforming to a first DMT standard based on a first set of tones, comprising (a) an analog-to-digital (A/D) converter and one or more filters configured to sample and filter the received DMT signal to generate a filtered, sampled DMT signal; and (b) additional components configured to further process the filtered, sampled DMT signal based upon a second DMT standard based on a second set of tones different from the first set of tones.